1. Field of Invention
This invention relates to conducted electromagnetic interference (EMI) testing and more particularly in a preferred embodiment to decoupling apparatus and methods used to determine EMI sources and receptors within electrical equipment.
2. Description of the Prior Art
Various techniques have been employed to detect, measure and then suppress EMI in sensitive electrical equipment or test items. Interference or susceptibility detection and measuring should be conducted with the test item operating as close to service conditions as possible. Also the test item normally is operated in its intended manner with anticipated inputs applied and its outputs normally loaded.
A problem exists with simulating service conditions and normal manner of operation. To bring a test item out of its normal operating environment and to place it on a bench for a test, an actual duplication of operation of the test item seldom occurs. For example, if in a vehicle equipped with an engine control system that includes a microcomputer as a controller, assume it is desired to use a particular portable mobile two-way radio. Discovering that the radio works when the engine is cutoff but doesn't work well while the engine hs running, the engine control system is then removed from the vehicle and placed in a bench test environment for study. A simulator (support for the engine control system) is used to make the control system work as if it is in the vehicle. Also assume the system responds as if it is operating in the vehicle controlling what it is supposed to be controlling. Assume also, the engine control system generates the same interference that it was putting out before it was placed on the bench; but now the interference reacts with the simulator. By virtue of change in wiring, the coupling between harnesses, other components and the impedance and length of wires, the engine control system generally radiates and conducts a different amount of interference.
Efforts have been made to standardize bench test setups in order to gain data that approaches actual circumstances.
In prior bench tests under similar circumstances, line impedance stabilization networks (LISN's) have been recommended in a number of interference and susceptibility specifications, for insertion in power leads to offer something approaching a standard impedance to the radio frequency (RF) current from test items. The LISN's, as required by some military specifications introduce a standard 50 ohm power-source impedance for the test item so that conducted RF interference measurements can be compared to pass/fail limits without accounting for a source-impedance variable. But, however, in several LISN designs, a 5-microhenry coil is used, so the device is suitable for use from 150 KHz to 25 MHz. Over this range, the source-impedance varies from about 5 ohms at 150 KHz to 50 ohms at 25 MHz. It is not usable much above 25 MHz due to stray impedance. While it does furnish a standard impedance, it is not the impedance seen in the normal installation. It was never intended to be anything other than an A.C. power lines simulator.
Normally when trying to identify the potential of a device being an interference source, it is thought that this determination depends upon how you measured the interference emanating from source. This implies that different test processes produced different results for the same interference source. Realizing the above conditions exist, efforts were made toward devising interference measuring technique that didn't depend upon how the interference was measured.